the very purpose for the noted drives is so that you CAN go from one server to another.
do they work when you go back to Server A?
pick the newest drive and query THEIR support personel, or maybe they'd like to buy it back.
"unallocated" ususally refers to the drive's format info etc. which MAY become corrupted then misinterpreted at some point.
I would expect that the MBR is being corrupted also.

I recognize that the purpose for the drives is that you can go from one server to another, hence my question of why they drop dead when I do so. I get both an unallocated message and a "disk not formatted" message when they have been NTFS formatted and had loads of data on them. If it happened to 1 server with 1 drive I would pass it off as an anomaly, but with 3 servers and 3 separate and distinct drives it becomes more of an issue.

given your post I assumed a few things.
1) all 3 servers are running the same file system
2) all are running the latest updates (although, for the most part this shouldn't matter all that much)
did I make a judgement error?

the format type (NTFS) on all of the drives, internal as well as external "should" not be that much of an issue, if you were mixing (format type) them then probably.
I hardly think this is actually a hardware error, more than likely an OS error. however, you'll have a better chance getting a response from the hdwr. OEM than MS unless maybe you've an service contract w/MS?
don't come up with much on the subject via the search eng. but agree that the PNP hdwr. SHOULD not be that much of a problem.

Would you be able to use the diskmanager to solve this???

Could you connect the drives to the original server?

Assuming you could handle the content somehow, what happens if you format the drive with FAT32 - if it not already is...?

Yes all 3 servers are Windows 2003 with all appropriate patches and updates. If I move one external drive to a different server, it not only doesn't work on the new server, it doesn't work on the old server. And I've gotten a couple of different error messages. One, if I look in diskmanager, is that it is unallocated. Once it's allocated it now needs to be formatted, removing of course all data that was on there. So I stopped moving them around. But last week I was moving some data from the server to one of the externals and after about 4 of these moves, I got a prompt that the drive needed to be formatted. So all that data that got moved on there was gone. So I've reformatted, brought down the data from our online backup system and will just leave it until I replace it with one of those network enclosure drives and just be done with these externals on a server. And they all are formatted as NTFS. Once I get a stable network enclosure device, maybe I'll get the time to reformat with FAT32 and see if that makes a difference. Thanks for your suggestions.